The objective of this proposal is to investigate neuroendocrine mechanisms which control the ovulatory cycle of the hen (Gallus domesticus). Our approach will concentrate on two perspectives; regulation of the ovulation-oviposition cycle by the hypophysis and by the ovary, and regulation of hypophyseal LH release and ovarian steroidogenesis by prolactin (PRL). Specifically, experiments are designed to determine: 1) The origin (ovary or hypothalamus/hypophysis) of the ovulation-inducing stimulus, and the mechanism by which the stimnulus is propagated (stimulatory action or positive-feeback mechanism); 2) Whether hypophyseal sensitivity to LH-RH is altered relative to the stage of the sequence (a mechanism which could control the initiation and length of the sequence; 3) Whether termination of the preovulatory LH surge is the result of hypophyseal LH depletion and/or decreased hypophyseal sensitivity to LH-RH; 4) Whether the fluctuations of plasma PRL in the laying hen and related to phtoperiod (i.e.a daily rhythm) or to the ovulation-oviposition cycle (i.e. a rhythm of 24+ to 30h); and 5) Whether this PRL rhythm acts as a regulator of hypophyseal LG release and/or ovarian steroidogenesis. Blood samples will be drawn by an indwelling brachial vein cnnula, and plasma LG, PRL, progresterone, testosterone, estaratdiol-17 Beta and corticosterone will be measured by radioimmunoassay. The time of ovulation will be determined by digital palpation. Results of these studies will contribute to the overall knowledge of comparative aspects of the control of ovulation in vertebrates. More specifically, determination of the origin of the ovulation-inducing stimulus will facilitate investigations of molecular-biochemical mechanisms which control the relatively unique 24+ to 30 h "clock(s)" of the hen. Such knowledge lhas widespread implications in the study of daily periodicity and has application within the fields of physiology, pharmacology and human reproduction. In addition, these data will contribute to the understanding of mechanisms whichregulate the ovulation-oviposition cycle and control sequence length in the fowl.